Method of eliminating dislocations and lowering lattice strain for highly doped N+ substrates

ABSTRACT

A method for fabricating semiconductor substrates with resistivity below 0.02 ohm-cm is provided. This low resistivity is achieved by doping a silicon melt with a phosphorus concentrations above 1×10 18 . The silicon melt is also doped with a germanium concentration that is 1.5 to 2.5 times that of the phosphorus concentration and a stress and dislocation free crystalline boule is grown. Phosphorus in high concentrations will induce stress in the crystal lattice due to the difference in the atomic radius of silicon atoms versus phosphorus atoms. Germanium compensates for the atomic radius mismatch and also retards the diffusion of the phosphorus as the diffusion coefficient remains relatively constant with a doping of 1×10 18  to 1×10 21  atoms per cm 3 . This will retard phosphorus from diffusing into an overlying epitaxial layer and retard other layers formed on the substrate from being auto-doped.

BACKGROUND OF THE INVENTION

This invention relates, in general, to doping silicon substrates andmore particularly, to compensating for the affects high concentrationsof dopant have on substrates.

It is well known in the art that the performance of a semiconductordevice can be improved by employing epitaxial films formed on substrateswith low resistivity. The resistivity of a substrate crystal can bereduced by doping the substrate with donor or acceptor atoms which willmake the substrate either n type conductivity or p type conductivity.Typical dopant atoms used, however, do not have the same atomic radiusas the substrate crystal atoms. Silicon atoms have an atomic radius of1.18 Å and common dopant atoms such as phosphorus and boron have anatomic radius of 1.08 Å and 0.86 Å respectively.

The size mismatch between the dopant atoms and the silicon substrateatoms will induce strain in the crystal. This strain is a result of thesubstrate lattice contracting as the lattice is forced to compensate forthe high concentration of smaller dopant atoms. As the concentration ofthese dopant atoms increases, the lattice will continue to contractwhich will further reduce the lattice constant. Current commercialprocesses do not distribute the dopant uniformly. The radial and axialnon-uniform dopant distribution will generate internal stress. Thisstress will be replicated and amplified by an overlying epitaxial layerand as the thickness of the epitaxial layer increases so too will thestrain in the layer.

A second difficulty with forming epitaxial layers on a substrate arisesfrom misfit dislocations at the substrate interface. In growing alightly doped phosphorus n- epitaxial layer on top of a heavily dopedphosphorus n+ substrate, the lattice parameter of the n- layer is largerthan the lattice parameter of the n+ substrate. Therefore, a layer ofmisfit dislocations is generated between the n- and n+ layers. Thesedislocations will hinder the performance of a semiconductor device asthe dislocations contribute to junction leakage

A high dopant concentration will also pose problems due to the presenceof a diffusion gradient. The high concentration of dopant in thesubstrate will naturally want to diffuse to areas of lower concentrationsuch as to the overlying epitaxial layer with a lower dopantconcentration This is likely to drive dopant atoms into the epitaxiallayer or auto-dope subsequent layers formed over the substrate.

Accordingly, in order to form n type substrates with resistivities below0.01 ohm-cm it is necessary to dope a substrate with phosphorusconcentrations above 4.5×10¹⁸ atoms per cm³. A method is necessary tocompensate for the lattice strain created by dopant atoms and retarddopant atoms from diffusing into layers overlying the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the lattice constant as a function ofphosphorus dopant concentration; and

FIG. 2 is a graph demonstrating the improvement seen in the diffusioncoefficient with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Previously known methods for reducing the sheet resistivity of siliconsubstrates have relied on doping the substrate with a n type or p typedopant. Unfortunately this approach is not without its limitations. Mostdopants used in silicon semiconductor processing have an atomic radiusthat is different than the atomic radius of silicon. As a result, asphosphorus or boron is added, the silicon crystal contracts since thedopant atoms are smaller than the host silicon atoms. This contractionwill induce strain in the lattice.

Stress is also created when an epitaxial layer is deposited on asubstrate that has a different lattice parameter. Such is the case whenan epitaxial layer is deposited on a heavily phosphorus doped n+substrate. As a result of the mismatch in lattice parameters,dislocations will be created along the interface of the substrate andoverlying epitaxial layer. These mismatch dislocations will hinder theperformance of a semiconductor device formed in the epitaxial layer asthey will increase junction leakage of the device.

This stress will increase as the thickness of the epitaxial layer isincreased. Stress will warp the substrate which hampers thephotolithography steps common to silicon processing. Highly dopedsubstrates also have a tendency to diffuse dopant into the epitaxiallayer or auto-dope overlying layers during subsequent processing.

One previously known method for correcting lattice mismatch strainbetween heavily boron doped p+ substrates and lightly doped epitaxiallayers was taught by Lin in U.S. Pat. No. 4,769,689 which was issued onSep. 6, 1988. In this particular application, germanium was added to thesubstrate in a concentration equal to approximately 8 times theconcentration of boron which was used to dope the substrate. This methodonly applies to boron doped substrates with concentrations higher than0.002 percent atomic weight and does not address issues associated withdiffusion of boron dopant into the epitaxial layer overlying thesubstrate. This previously known method for doping a substrate withboron and germanium only reduces the substrate resistivity from 0.01ohm-cm to 0.002 ohm-cm.

The use of germanium to compensate silicon substrates that are heavilydoped with boron does have limitations. As reported by Aronowitz et al.in "P-type Dopant Diffusion Control in Silicon Using Germanium" whichwas published in the June 1991 issue of the Journal of theElectrochemical Society, the presence of germanium will enhance andaccelerate the diffusion of boron atoms which are on substitutionallattice sites in the silicon crystal. Thus in order to fabricatesubstrates with low resistivities via high concentrations of boron,germanium cannot be used to compensate the lattice. The accelerated rateof diffusion will cause the boron to auto-dope more into the overlyingepitaxial layer or subsequent layers formed on the substrate.

In the present invention, a method is provided for achieving substrateresistivities that are two orders of magnitude lower than taught by Lin.The present invention also provides benefits not provided in the patentby Lin such as eliminating the dislocations between a heavily doped n+substrate and a lightly doped n- epitaxial layer and retarding thephosphorus diffusion rate. A highly doped n+ substrate is fabricatedusing a first dopant such as phosphorus with concentrations of 1×10¹⁸ to1.1×10²⁰ atoms per cm³ to reduce substrate resistivity below 0.02ohm-cm. With such a high concentration of atoms that have an atomicradius smaller than the crystal host atom, the lattice will contract.FIG. 1 quantifies the amount of lattice contraction as a function ofphosphorus doping (shown as line 10). The y-axis shows the latticeconstant of the crystal in Å versus the x-axis which is theconcentration of dopant atoms in terms of the number of phosphorus atomsper cubic centimeter.

To compensate for lattice contraction due to high concentration ofdopant atoms, a second dopant of germanium is added to the lattice.Germanium has an atomic radius of 1.28 Å which will compensate for thesmaller phosphorus atoms. One atomic percent of germanium in siliconwill cause the lattice to expand 0.0022 Å. For example, to fabricate a ntype semiconductor substrate with a resistivity of 0.001 ohm-cm,phosphorus is added to the substrate with a concentration of 7.38×10¹⁹atoms per cm³ which is 0.148 atomic percent. This concentration ofphosphorus will cause the lattice to contract 0.00157 Å and require agermanium concentration of 0.71 atomic percent to compensate for latticestrain. This concentration of germanium is 4.8 times that of thephosphorus. Experimental and theoretical calculations suggest that thepreferred germanium concentration is approximately 1.5 to 2.5 times thatof the phosphorus concentration.

A method will now be provided to fabricate the above mentionedembodiment of the present invention. A substrate is fabricated using thetraditional Czochralski technique to pull a dislocation free singlecrystal silicon boule from a molten mixture. Such a technique wasdescribed in U.S. Pat. No. 4,200,621 which was issued to Liaw et al. onApr. 29, 1980 and is hereby incorporated herein by reference. Thepresent embodiment is not limited to the weights of the followingexample. A stress free crystalline boule can be formed by maintainingthe proper proportions of constituents. The molten mixture is formed byplacing approximately 18 Kgrams of polysilicon chunks with 95 grams ofgermanium chips in a quartz crucible and heated to the melting point of(1420° C.) in an argon atmosphere. Then 60 grams of phosphorus is placedin a quartz beaker with an inverted bell jar on its top. The beaker ofphosphorus is placed above the molten silicon surface such that a flowof phosphorus vapor is directed at the melt and allowed to diffuse intothe molten silicon. As is well known in the art, the semiconductorsubstrate wafers are formed from the crystalline boule by sawing thewafers with the desired thickness.

By using a germanium concentration that is 1.5 to 2.5 times theconcentration of the phosphorus, several issues associated with a highlydoped substrate are addressed. Since germanium has a larger atomicradius than silicon, germanium compensates for the contraction of thecrystal lattice due to the presence of smaller phosphorus atoms. Thisrenders the substrate essentially stress free and will permit anepitaxial layer of a first conductivity type such as n type or a secondconductivity type such as p type to be formed overlying the substrate.An epitaxial layer of any thickness can be formed on the substratewithout inducing strain or bowing of the substrate.

The addition of germanium to the substrate mixture also addressesproblems of diffusion when doping a substrate with phosphorus above1×10¹⁸ atoms per cm³. Concentrations of phosphorus of this magnitudewill have a high diffusion rate which will tend to diffuse dopant fromthe highly concentrated substrate to the lower concentration epitaxiallayer. The amount of phosphorus that will diffuse from the substrate isproportional to the diffusion coefficient of the dopant atoms. FIG. 2demonstrates the improvement gained by doping the substrate withgermanium by plotting the diffusion coefficient (y-axis) as a functionof the number of phosphorus atoms per cubic centimeter (x-axis).Previously without germanium, the high phosphorus concentration willdiffuse with a diffusion coefficient value shown as line 11. In thepresent invention, the addition of germanium to the substrate impedesthe phosphorus diffusion and the diffusion coefficient remains constantwith increasing phosphorus concentration shown as line 22 in FIG. 2.

In a paper published by Matsumoto et al. in the November 1978 issue ofJournal of Electrochemical Society entitled "Effects ofDiffusion-Induced Strain and Dislocation on Phosphorus Diffusion intoSilicon" a method for retarding the surface diffusion of phosphorus waspresented. In their experimentation, Matsumoto et al. doped regions ofan already grown silicon crystal using liquid phosphorus sources. Thediffusion of phosphorus atoms out of these doped regions createddislocations on the surface of the silicon substrate. By doping thesubstrate from the surface with a liquid germanium source, the surfacediffusion of the phosphorus was retarded. This technique does reduce thenumber of surface dislocations, however, dislocations will form in thesilicon substrate due to the migration of the germanium atoms into thesilicon lattice. In the present invention, the germanium dopant is addedto the silicon lattice during the crystal growth instead of relying onsurface diffusion with liquid sources. By adding the germanium to thesilicon crystal during the boule growth, the present invention providesa method for forming dislocation free substrates that avoids the formingof dislocations from liquid diffusion of germanium.

An additional benefit of the present invention which was not offered bypreviously known methods, is improvement in junction leakage. Mismatchdislocations present in the substrate crystal or present at theinterface of the substrate and epitaxial layer, increase the junctionleakage of semiconductor devices. The number of dislocations in theepitaxial film can multiply and migrate from the substrate duringsubsequent thermal processing. The presence of germanium in thesubstrate will produce substrates that are relatively free ofdislocations. Therefore, the addition of germanium to the substrate willreduce junction leakage.

The present invention provides an embodiment that allows for the growingof a n- epitaxial layer on a heavily doped substrate by addressing theissues associated with a substrate with phosphorus concentrations above1×10¹⁸ atoms per cm³. By adding germanium in a concentration that is 1.5to 2.5 times the concentration of phosphorus, it is possible to producesubstrates that are both relatively stress and dislocation free. Thegermanium dopant corrects the lattice mismatch stress created byphosphorus atoms which have an atomic radius that is different than thesilicon crystal substrate.

The germanium also addresses the diffusion problems associated with suchhigh doping concentrations. By occupying substitutional lattice sites inthe crystal, germanium will keep the diffusion coefficient constant upto phosphorus concentrations of 1×10²¹. It should also be appreciatedthat germanium is isoelectric in a silicon crystal and will not affectthe conductivity of the n type substrate.

We claim:
 1. A method for fabricating a highly doped n typesemiconductor substrate essentially free of dislocations and latticestrain comprising the steps of:forming a melt from silicon chunks and afirst dopant having a first concentration; doping the melt with a seconddopant having a second concentration, wherein the first dopantcompensates for the lattice strain created by the second dopant and thesecond dopant lowers resistivity of the melt; growing a crystallineboule from the melt; and forming substrates from the crystalline boule.2. The method of claim 1 wherein the first dopant consists essentiallyof germanium.
 3. The method of claim 1 wherein the second dopantconsists essentially of phosphorus.
 4. The method of claim 1 wherein thefirst concentration of the first dopant is approximately 1.5 to 2.5times the second concentration of the second dopant.
 5. The method ofclaim 1 wherein the second concentration of the second dopant reducesthe resistivity of the highly doped n type semiconductor substrate toless than 0.02 ohm-cm.
 6. The method of claim 1 wherein the step ofdoping the melt is accomplished by placing a quartz beaker with aninverted bell jar containing a source of the second dopant.